In the discussion of the state of the art that follows, reference is made to certain structures and/or methods. However, the following references should not be construed as an admission that these structures and/or methods constitute prior art. Applicant expressly reserves the right to demonstrate that such structures and/or methods do not qualify as prior art.
Yttria Tetragonal Zirconia Polycrystalline (YTZP) materials have emerged as a high-strength framework material for dental prostheses (single-units up to multiple unit bridges). However due to its inherent white color, often the esthetics of the finished restoration is inferior to what is achievable with other all-ceramic systems.
Currently there are two predominant commercially available methods to deal with the stark white color of zirconia. In the one method, the color of the zirconia is “hidden” by applying either a layer of stain or liner. The other method entails shading the zirconia by immersion in, or painting with, coloring solutions while in the pre-sintered state. Coloring with a stain and/or applying a liner involves an extra fabrication step and lowers translucency. Shading with a coloring solution similarly requires the extra step of dipping or painting, and extra time to dry before sintering. Also, this method is deficient as the color of the final sintered framework often is not uniform.
An alternative method is to use porous zirconia blocks that are preshaded to the desired coloration. Such blocks only need to be fired after any machining, thus eliminating the coloring with solutions step. As the fully sintered frameworks emerge from the furnace already shaded, the stain/liner step can be eliminated. Additionally, the color of the sintered frameworks is characteristically uniform, which is another advantage over the shading with coloring solution method.
A finished dental restoration should match the color of the patient's teeth, i.e., it should be “tooth colored”. The colors of human teeth appear to range from a light almost white-tan to a light brown, and occupy a very specific color space. This color space can be described by the commonly used CIE (Commission Internationale de I'Eclariage) L*, a*, b* conventions, which represents colors in a three-dimensional Cartesian coordinate system. L*, or “value”, is a measure of luminance or lightness, and is represented on the vertical axis. The a*, b* coordinates, are a measure of chromaticity and are represented on the horizontal coordinates, with positive a* representing red and negative a* representing green, and positive b* representing yellow and negative b* representing blue. U.S. Pat. No. 6,030,209, which is incorporated herein by reference, presents the CIE L*, a*, b* color coordinates of tooth colors represented by the Vita Lumen® shade guide system manufactured by Vita Zahnfabrik (i.e., it presents the color space of tooth colors). Herein, “tooth color” is taken to mean CIE L*, a*, b* color coordinates that fall within, or very close to, this color space.
U.S. Pat. No. 6,713,421 appears to describe yttria-stabilized zirconia dental milling blanks that are formed with 0-1.9 wt. % coloring additives. The composition described therein includes 0.1 to 0.50 wt. % of at least one oxide of aluminum, gallium, germanium and indium for the purpose of lowering the sintering temperature and increasing stability and hydrolytic resistance in the densely sintered state. However, the addition of aluminum oxide (alumina) to zirconia also often results in discrete alumina inclusions distributed throughout the microstructure. This occurs in part due to the low solubility of alumina in zirconia. Further, it presents a particular disadvantage for dental applications because alumina inclusions can lower the translucency of the zirconia since the refractive index of alumina, 1.77, differs considerably from that of tetragonal zirconia, 2.16. Thus, it is desirable that dental zirconia is devoid of any alumina inclusions. A means to achieve this is to minimize, or eliminate, the alumina addition, thereby minimizing the potential for the alumina inclusions in the final microstructure.
In U.S. Pat. No. 6,713,421 the blanks are made from powders or granules that have been doped with the various oxides via a solution followed by a co-precipitation method. The cited advantage of this method is that the various oxides are distributed homogeneously throughout the powder. However, the field of dental restoratives requires many shades (e.g., 7 zirconia core shades as per LAVA, 16 Vita Classic shades, etc.), and having to prepare so many individually shaded powders or granules can be cost-prohibitive.
Yet another disadvantage of U.S. Pat. No. 6,713,421 is that it requires relatively large amounts of the preferred coloring oxides, iron oxide and erbium oxide. This is revealed by the Preparation Example 1 cited which teaches adding 0.2 wt. % iron oxide+0.38 erbium oxide (0.58% total) to color 3YTZP. Although the patent does not indicate if this resulted in a tooth color, it can be inferred from U.S. Pat. No. 5,219,805, which appears to disclose coloration of yttria-stabilized zirconia for dental bracket applications using combinations of Fe2O3, Er2O3, and Pr6O11, that even higher Fe2O3 and Er2O3 concentrations are necessary to achieve tooth coloration. For instance, according to the examples given in U.S. Pat. No. 5,219,805, up to 1.0 mol % Er2O3 (3.0 wt. %) additive is required to achieve dental brackets “having color tone similar to ivory-colored teeth”. Additionally, up to 0.2 mol % Fe2O3 (0.25 wt. %) is required to achieve tooth colors, which although less than the 1 mol % Er2O3 required, is a considerable amount. As such quantities are significant, they can have a negative effect on other properties of the resulting YTZP cores, such as on strength, weibull modulus, hydrolytic resistance, and grain size.
Additionally, it has been observed that Er2O3 additions to 3Y-zirconia, of 0.2 wt. % or greater, results in sintered bodies that fluoresce a dark yellow under ultraviolet (UV) lighting. This is inappropriate for a dental framework, which under UV, ideally, should fluoresce bluish-white to mimic that of natural teeth. Less ideally, the framework should not fluoresce at all in the visible light range. In the latter case fluorescence is typically imparted to the final restoration by the overlay porcelains. The shortcoming of an inappropriate fluorescence is overcome by the present invention.
The prior art also shows Cr additions result in green or brown coloration. For example, U.S. Pat. No. 3,984,524 appears to describe olive coloration of cubic zirconia with addition of 0.1 to 2 wt. % Cr2O3, U.S. Pat. No. 4,742,030 appears to describe green coloration of 5 mol % yttria-stabilized zirconia with addition of 0.7 wt. % Cr2O3, and brown coloration with addition of 0.2 wt. % Cr2O3, respectively.
French patent publication 2,781,366 and Cales et. al. (“Colored Zirconia Ceramics for Dental Applications,” Bioceramics Vol. 11, edited by R. Z. LeGeros and J. R. LeGeros; Proceedings of the 11th International Symposium on Ceramics in Medicine; York, N.Y.; November 1998) appear to identify a number of colorants, and was reportedly successful in achieving some of the Vita shades in 3YTZP by using combinations of Fe2O3, CeO2 and Bi2O3. However, their choice of colorant oxides is a drawback as they are required in fairly large amounts to achieve some of the desired shades.
U.S. Pat. No. 5,656,564 appears to teach coloration of zirconia for dental bracket applications using with combinations of Er2O3 and Pr6O11. The sintered zirconia-based ceramic is produced by a procedure generally including combining constituents in solution, precipitating, calcining, pressing, and sintering.
U.S. Pat. No. 5,011,403 appears to describe coloration of zirconia dental brackets using combinations of one or more of oxides of Fe, Ni and Mn added to a Zr-based powder.
U.S. Pat. No. 6,709,694 appears to describe the use of solutions for coloring of pre-sintered zirconia dental frameworks by immersion, painting or spraying using a metal ion coloring solution or metal complex coloring solution that is applied to a presintered ceramic, followed by sintering to form a translucent, colored dental ceramic. The claimed ions or complexes are of the rare earths elements or subgroups II and VIII, with an action time of under two hours, and maximum pre-sintered zirconia diameter and height of 10 and 7 mm, respectively. However, this method is not ideal as the color of the final sintered frameworks often are not uniform and the process requires the extra steps of applying the solutions and drying prior to sintering.
The development of pink coloration in zirconia by Er additions is described in (i) P. Duran, P. Recio, J. R. Jurado, C. Pascual and C. Moure, “Preparation, Sintering, and Properties of Translucent Er2O3-Doped Tetragonal Zirconia,” J. Am. Ceram. Soc., vol. 72, no. 11, pp. 2088-93, 1989; and (ii) M. Yashima, T. Nagotome, T. Noma, N. Ishizawa, Y. Suzuki and M. Yoshimura, “Effect of Dopant Species on Tetragonal (t′)-to-Monoclinic Phase Transformation of Arc-Melted ZrO2—RO1.5 (R=Sm, Y, Er, and Sc) in Water at 200° C. and 100 MPa Pressure,” J. Am. Ceram. Soc., no. 78, no. 8, pp. 2229-93, 1989. Additions of CoO, Fe2O3 and Cr2O3 combinations to yttria-stabilized zirconia are known to impart a blue color in the final sintered zirconia bodies, as apparently described in Japanese patent publication 2,145,475. Additions of one or both of the colorants, Ni oxide and Cobalt oxide, to yttria-stabilized zirconia have been shown to result in a purplish colored sintered body, as apparently described in U.S. Pat. No. 5,043,316.
Japanese patent publication 3,028,161 appears to describe the preparation of colored zirconia by the steps of: (1) mixing zircon-based pigment with partially stabilized zirconia containing Y2O3, MgO, etc., (2) molding and (3) sintering to provide a colored zirconia sintered product.
Many of the aforementioned coloring additions can negatively affect not only mechanical properties, including strength and fracture toughness, but also isotropic shrinkage and final sintered density. This can happen for a number of reasons including: (1) loss of fracture toughness from a lowering of the “transformation toughening” effect as a result of the over-stabilization of the tetragonal phase by the additive (either chemically, or by grain size reduction) thereby hindering the transformation from the metastable tetragonal phase to monoclinic phase that is necessary for the toughening to happen, (2) loss of strength due to spontaneous microcrack formation that can result if grains grow too large because of the additive, and, (3) loss of strength due to the formation of strength-limiting pores in the microstructure due to the additive. This last reason is what Shah et al. (K. C. Shah, I. Denry and J. A. Holloway, “Physical Properties of Cerium-Doped Tetragonal Zirconia,” Abstract 0080, Journal of Dental Research, Vol. 85, Special Issue A, 2006) attribute the significant loss of strength, down to 275±67 MPa, for 3YTZP materials that were colored using Ce salts. Additionally, they observed that strength decreased linearly with the concentration of the coloring additive, Ce.
The problem of formation of coarse pores, along with grain growth, in colored zirconia sintered compacts has also been recently recognized in JP 2005289721.
It is also important to recognize that only certain combinations of coloring agents in certain proportions will enable the matching of the color of a dental article so as to match the desired natural tooth color, e.g., A, B, C, D of the Vita classic shade guide, and Chromoscop® universal shade guide.
Thus, it would be extremely beneficial to have pre-shaded YTZP blocks or blanks that sinter isotropically to full density and that yield the required variety of shades consistently and without compromise in strength, fracture toughness, and reliability or Weibull modulus.